While KEO's employees and exact history are unknown, they were implied to be founded in 1864 when the British Empire colonized Kyrat. After the country got its independence from the British Empire, presumably after the Second World War due to the amount of weapons styled from that time period, KEO was allowed to stay and soon Kyrat's post-colonial economy flourished as they became dependent on gold mining.
During the 1960's, KEO's mines began to dry up, causing KEO to lose funds. Soon afterwards, the mines were completely exhausted, so KEO went bankrupt and Kyrat's economy spiraled into free fall. This lead to chaos and civil unrest during the 1970's and fueled the Nationalists cause, eventually resulted in First Kyrati Civil War.
The Royal Guard now occupies the mines ever since Kyrat came under Pagan Min regime grip in 1987. It's implied on the logo that Pagan Min regime fully revived the company in 2002. Amita claims that the mines are still exhausted and are stripped bare, but new gold veins are still being found, as seen when Fullan bee farm got burned down by a corrupt Royal Guard Heavy Flamer Commander until being stopped by Ajay Ghale. Nevertheless, KEO assets such as trucks are still being used by Royal Guard to deliver supplies, and miners are still working. Being founded in 1864, by the time of Ajay's return in 2014, the company was 150 year old.